Idiopathic thrombocytopenic purpura (ITP), neonatal purpura (INT) post-transfusion purpura (PTP), drug-purpura and thrombocytopenia associated with infection or altered immune states are the major immunologic thrombocytopenias. Antibody reactions in these disorders are relevant to autoimmunity, histocompatibility, malignant surveillance, alloimmunity, pathogenicity of antigen- antibody complexes and cellular immune injury generally. We have identified internal platelet proteins that bind immunoglobulins by Western blot (WB) and have purified and characterized a 95 kD receptor by anion exchange chromatography and HPLC molecular sieving and chromatofocusing. Platelet alloantigens responsible for post transfusion purpura were found to circulate in normal plasma and to be adsorbed in sufficient amounts as antigen-antibody complexes to account for thrombocytopciua in transfused subjects. Anti-cardiolipins (ACLs) have been found on platelets of lupus erythematosus (SLE) patients but do not correlate with serum ACLs or with platelet counts. Although 30% of 90 SLE patients had elevated platelet-associated IgG (PAIgG) and other positive serologic tests for presumed anti-platelet antibodies their autologous platelet survivals with III-In were normal. On the basis of 53 cases of alloimmune neonatal thrombocytopenia the value was established of treating mothers antenatally with adrenocortico steroids and I.V. IgG that cross the placenta to prevent fetal hemorrhage at birth. A microtiter EIA was developed to facilitate phenotyping platelet antigens for matching transfusions in a Blood Bank setting.